New laptop - should I go for XP Pro?

I'm on the verge of buying a HP Laptop and can pay an additional &89 to upgrade it to XP Pro. The laptop will be in addition to my main PC which I hope to link via a wireless network.

My main PC has XP Pro but an invalid product key number. If I pay the extra £89 which presumably will give me a genuine XP product key number will I be able to use that key number on my main PC to get updates from Microsoft?